In the prior art, as ferromagnetic powder of a magnetic recording medium, fine powder of .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, and these adsorbed or doped with cobalt ions, etc. have been used. In recent years, as the video tape, audio tape have been demanded to have higher performances, high signal density and high reproduction output in short wavelength recording are required, and to cope with this trend, ferromagnetic metal fine powder composed mainly of Fe, Fe-Co, or Fe-Co-Ni, have been used as ferromagnetic powder. However, such ferromagnetic metal powder has small crystallite size and yet very great magnetic moment, and therefore the above particles are liable to be agglomerated, with the result that it has become very difficult to disperse uniformly ferromagnetic metal fine powder into a binder.
For such problems, in order to enhance affinity of the binder for ferromagnetic powder, for example, binders containing caboxylic groups, sulfonic acid bases, etc. in the molecular structure have been proposed (JP-A-57-92422) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but due to insufficient dispersibility of ferromagnetic metal fine powder, there have been involved the drawbacks that the surface characteristic of the magnetic coating was inferior, the residual magnetic flux density and the squareness ratio were unsatisfactory, and also powder drop-off is liable to occur resulting in inferior durability.
Accordingly, as a magnetic recording medium excellent in dispersibility of ferromagnetic powder, durability of magnetic layer and surface gloss, JP-A-59-40320 has proposed a combination of a vinyl chloride type binder and a polyurethane type binder having high dispersing function with hydrophilic groups introduced therein. This describes that the polyurethane type binder with hydrophilic polar groups introduced therein contributes to enhancement of the dispersibility of the ferromagnetic powder and the durability of the magnetic layer, while the vinyl chloride type binder with hydrophilic groups introduced therein contributes to enhancement of the dispersibility and the surface gloss of the ferromagnetic powder. Surely, by such combination, the improvement effects can be seen in these respects, but no improvement could be effected with respect to calender roll contamination, clogging, etc. JP-A-60-235814 proposes a binder obtained by copolymerization of vinyl chloride with a monomer having a strong acid group containing sulfur or phosphorus and a monomer having X-OH group (wherein X is an organic residue) in order to improve the characteristics of a vinyl chloride type copolymer. This improves dispersibility and gloss by having a strong acid group such as sulfonic acid, and also due to the presence through an organic group without direct bonding to the main chain as in vinyl alcohol, crosslinking reaction with isocyanate can be effected sufficiently, whereby durability, running performance are stated to be improved. However, even by use of such binder, calender roll contamination and clogging cannot be improved sufficiently.
Further, for effecting improvement of dispersibility of ferromagnetic metal fine powder and smoothening of the coating surface, there have been proposed the techniques of adding a surfactant, treating ferromagnetic powder with silicone oil, etc., adding a silane coupling agent having reactivity with the binder, etc., but none of these techniques has exhibited sufficient effect, and it has been demanded to have a binder exhibiting further excellent dispersibility for the ferromagnetic metal fine powder to be used for the magnetic recording medium for high density recording.